Spatial variability in the response of lower trophic levels after carp exclusion from a freshwater marsh

Abstract:

Large common carp (Cyprinus carpio >30 cm) were excluded from a turbid, eutrophic coastal marsh of Lake Ontario with the construction of a fishway at the outlet. The marsh was sampled intensively for 2 seasons prior to (1993, 1994) and following (1997, 1998) carp exclusion to study changes in water quality and shifts in community structure of phytoplankton and zooplankton. Samples were collected from May to September in three habitats: open water, vegetated (cattail beds) and sewage lagoon. In the first year after carp exclusion, mean seasonal water turbidity decreased at all sites by 49–80%; this was accompanied by growth of submergent plants in shallow, sheltered areas including the vicinity of cattails at the vegetated site. This drop in turbidity was not significant in the second year after exclusion at the open water and lagoon sites, with turbidity levels declining by only 26–54% of 1993–1994 values; only the vegetated site showed a sustained decrease in turbidity and persistent growth of submergent plants. At the vegetated site, increased clarity was concurrent with a significant reduction in edible algal biomass and an increased representation of large zooplankton grazers and substrate-associated cladocerans. At the open water site, a spring clear-water phase was evident during the first year of exclusion and this coincided with the unusual appearance of a large population of Daphnia. Compared to the other sites, the lagoon remained relatively turbid throughout the study. Results of this study indicate that the response of lower trophic levels to biomanipulation was variable from site-to-site and contributed to the co-existence of two alternative states in the marsh. In vegetated areas, water clarity was maintained by a positive feedback system between zooplankton and submergent macrophytes in the first 2 years following exclusion. We suggest that both benthivore removal (to reduce bioturbation) and planktivore reductions (to produce top down effects) were required to produce clear water and allow submersed macrophyte growth. Although carp removal likely contributed to a 45% reduction in turbidity, an unusual climactic event in 1997, resulting in delayed fish spawning in the marsh, temporarily reduced zooplanktivory and favoured zooplankton grazing-induced water clarity improvements.